Gas sensing is an important science with broad reaching implications throughout society. Gas sensors are used to monitor gas levels for safety, environmental impact and process control. Two economically important sensor types are for monitoring of household pollutants, such as carbon monoxide or explosive hydrocarbons, and oxygen sensors for combustion monitoring and control in automobile engines. The primary problem with oxygen sensors today is their lack of selectivity and limited sensitivity near lean bum conditions. These sensors primarily are based on a Nernstian response to oxygen partial pressure, and competitive surface reactions from many gases play a role in the sensor's response.
One approach to this problem is the use of amperometric sensors, which rely on the pumping of anionic oxygen species through an electrochemical membrane and measuring the associated electrical current through an electrical circuit. The advantage here is that the current measured has a direct linear relationship to the oxygen content in the gas, so long as the sensor is not overwhelmed by the overall oxygen gas concentration. In essence, the sensor must react with all interacting oxygen species in order to accurately count them. This mode is called the current-limiting mode for the sensor since the current measured is limited by the oxygen concentration. The best method of dealing with higher oxygen concentrations to date has been the diminishment of oxygen concentration near the sensor surface vis-à-vis a separate chamber, which is oxygen limited through the use of a diffusion limiting hole or separate oxygen pumping mechanism prior to the chamber. Neither of these prospects has received commercial recognition.
With continuous improvements and stringency in environmental regulations and advances in emission control technology, there is an intense demand for low-cost, high sensitivity gas sensors for better control of combustion in order to minimize pollutant emission while improving energy efficiency. One of the most important gas sensors is the solid-state oxygen sensor for control of the air-to-fuel ratio in automobiles, furnaces, and other combustion processes. While potentiometric oxygen sensors have been widely used for control of stoichiometric combustion, they are not adequately sensitive to changes in oxygen concentration when the partial pressure of oxygen in a sample gas is too close to that of a reference gas, typically air, because of the logarithmic response. On the other hand, an amperometric, or a limiting-current type, oxygen sensor exhibits a linear dependence on oxygen concentration in the sample gas. Amperometric sensors are, therefore, more suitable for control of lean-burn combustion.
For a traditional amperometric oxygen sensor, a porous ceramic layer, or a cap with a laser-drilled hole, is used as a diffusion barrier to control the inflow of oxygen. The characteristics of such a sensor depend critically on the microstructure of the diffusion barrier, or the size of the hole. The disadvantages associated with this design include: the pore or hole dimension is difficult to control; and (ii) the pores or hole can be readily blocked by particulates in the sample gas to be monitored.
To overcome these difficulties, mixed-conducting ceramic membranes have been used as the diffusion barrier for amperometric sensors, as described, for instance, in U.S. Pat. No. 5,543,025 to Garzon, et al. To date, however, the mixed conductors typically have been formed of lanthanum strontium manganese oxide (LSM), lanthanum strontium cobalt oxide (LSC), and terbia—yttria stabilized zirconia (Tb—YSZ). The stability of these mixed conductors is questionable and the reliability of a solid-state gas sensor depends mainly on the stability of the sensing components, particularly the one in contact with exhaust. For example, the stability and reliability of a sensor based on a mixed-conducting membrane may depend on the stability of the dense mixed-conductor membrane exposed to exhaust containing various pollutants at temperatures up to 1100° C.
It is well known that LSM and LSC are not very stable in gases containing unburned hydrocarbons and sulfur-containing compounds at high temperatures. These mixed conductors may undergo irreversible structural changes when exposed to unburned hydrocarbons. Further, they may readily react with sulfur-containing gases at temperatures up to 1100° C., forming reaction products at the surfaces that may alter the electrical properties of the materials. Accordingly, the performance of a sensor based on these mixed conductors may change during the course of operation, leading to drift in sensor output (or lack of stability), and even to sensor failure.
In addition to the chemical stability, the transport properties of the mixed conductors used as the diffusion barrier must not change significantly over the oxygen partial pressure range of interest in order to achieve wide-range oxygen detection. Unfortunately, the transport properties of LSM, LSC, and Tb—YSZ are known to change significantly with partial pressure of oxygen (Po2), implying that sensors based on these materials may change sensing characteristics under these conditions. Further, these changes are often very slow, causing a slow drift in sensor responses. Therefore, it is desired that the mixed conductors used as the diffusion barrier have excellent stability under operating conditions to achieve stability, reliability, and reproducibility, and fast response.
Preparation of dense ceramic membranes on porous electrodes or substrates is an important step in fabrication of high-performance solid-state ionic devices or electrochemical systems such as solid oxide fuel cells (SOFCs), gas sensors, membrane reactors for gas separation or electrosysnthesis, and reformers for the processing of hydrocarbon fuels. In these applications, generally thin ceramic membranes are supported by porous substrates (or porous electrodes) since the electroactive species and the reaction products must be able to transport to or away from the surfaces of the dense ceramic membranes.
Various film deposition techniques have been explored for preparation of dense ceramic membranes on porous substrates, including a variety of atomic-scale physical and chemical vapor deposition, sol-gel process, electrochemical vapor deposition, combustion chemical vapor deposition, and more traditional particle deposition techniques. The atomic-scale deposition techniques involving a vapor phase or a solution often face difficulties either in stoichiometry control or in preventing the gas or the solution from filtration into the porous substrates. On the other hand, the particle deposition techniques such as electrophoretic deposition, colloid coating, screen-printing, tape casting, and tape calendering often have difficulties in achieving the required density or desired thickness. Further, and in particular, many of these film deposition techniques are complex, difficult to control, and expensive. In fact, it is the cost of fabrication that makes the commercial realization of many advanced technologies unaffordable. For example, while the existing SOFC technology has demonstrated much higher energy efficiency with virtually no pollutant emission over conventional energy technologies, the cost of the current SOFC systems is prohibitive for wide commercial applications. The cost of SOFC stacks is a major cost of fabrication.
Therefore, there is a need for improved oxygen sensors, systems and methods which address these and other shortcomings of the prior art.